(1) Field of the Invention
The present invention relates to a polyethylene terephthalate (hereinafter referred to as PET) film, process for the production thereof and magnetic recording medium containing the same as a base. Particularly, the present invention relates to a PET film having quite high mechanical strengths, both in longitudinal and transverse directions, excellent dimensional stability and low heat shrinkage, process for the stable production thereof and the magnetic recording material such as a magnetic tape containing this film as a base.
(2) Description of the Prior Art
As for PET films having high mechanical strengths, both in longitudinal and transverse directions, and process for the production thereof, there has been known a process wherein a biaxially oriented film is successively restretched biaxially in longitudinal and transverse directions as disclosed in the specification of U.S. Pat. No. 4,226,826. However, this process has defects that the mechanical strength, i.e. F-5 value of the film cannot be increased sufficiently and if the stretching ratio is increased so as to obtain a film having a high F-5 value, the restretchability becomes bad and heat shrinkage is increased. Further, there has been known a process wherein a biaxially oriented film is simultaneously restretched both longitudinally and transversely as shown in the specification of U.S. Pat. No. 4,234,532. However, this process also has defects that if the F-5 value is increased, the restretchability becomes bad to cause troubles such as breakage of the film in the process of the stretching of the film and heat shrinkage is increased. Therefore, when a film obtained by either of these known techniques is used as a base to provide a magnetic recording layer thereon, the base film is distorted or a uniform tone of an audio recording type or picture quality of a video tape cannot be obtained.
It has been impossible in the prior art to obtain a film having both satisfactory mechanical strength and dimensional stability. One of the characteristic features of the present invention is that the longitudinal stretching which is effected first is carried out under specific conditions to obtain a stretched film having specific properties, particularly birefringence and amorphous orientation factor and, accordingly, restretchability in the next transverse stretching and/or restretching step is improved and properties of the film thus obtained are far superior to those of films obtained in the prior art. Thus, the inventors have succeeded in obtaining a film having quite high strengths, and excellent dimensional stability such as low heat shrinkage.